This invention relates to power steering systems for material handling vehicles, and more particularly, to a self-centering, torque-sensing joint assembly disposed in a steering mechanism that detects operator steering intent for an electrically driven, hand-steered pallet truck regardless of angle of the steering mechanism.
Industrial material handling vehicles such as electric hand-steered pallet trucks are commonly found in warehouses, factories, shipping yards, and, generally, wherever pallets, large packages, or loads of goods are required to be transported from place to place. Pallet trucks typically include load bearing forks for lifting packages or pallets for transporting, an electric drive motor for propelling the truck, a steering control mechanism, and a brake.
A conventional steering mechanism includes a movable tiller arm mechanically coupled through a transmission housing to a steerable drive wheel. A control handle is connected to the tiller arm and may include speed, lift/lower, jog, and horn controls. To steer the pallet truck, an operator applies a force to the handle in the desired direction of travel. The steering force is transferred directly into the transmission housing via the tiller arm and the drive wheel is moved accordingly.
To facilitate manual steering, the tiller arm is generally several feet in length to provide sufficient leverage. Nevertheless the required steering effort can be greater in certain conditions when the tiller arm is necessarily oriented in a more vertical position, such as when negotiating within the confines of an over-the-road trailer or other limited-access location with a heavy load. Even when the tiller arm is oriented in a generally horizontal position, certain condition requires a larger than desired steering force, such as when the drive wheel is restrained by floor debris or a floor depression.
For these reasons, power steering assist, or “torque boost,” systems have been developed for electrical hand-steered pallet trucks to provide an amount of steering assist to aid the operator. In these systems, the amount of steering assist is typically based on a torque value produced when an operator applies a steering force to the handle and measured in a horizontal plane about a steering axis of the pallet truck. As such, the sensitivity and therefore, accuracy, of these torque sensors is dependent on the angle of the tiller arm. Accordingly, the same amount of steering force exerted by an operator will result in different torque measurements at different tiller arm angles with respect to a plane of sensitivity of the torque sensor. Conventional steering assist systems do not account for changes in measured torque value due to tiller arm angle and thus, deliver varied and inconsistent steering assist.
For example, when a pallet truck with a conventional steering assist system is operating in very close quarters, i.e., with the steering mechanism in a near-vertical orientation, the torque sensor is least sensitive and accurate. Thus, the measured torque value is a fraction of what would be measured when the steering mechanism is in a near-horizontal orientation. Because of the discrepancy, the steering assist provided to turn the drive wheel is quite small, regardless of the amount of operator steering force applied to the steering handle.
Another challenge for conventional power assist systems is that the motion required to steer a pallet truck changes from a lateral force applied to the handle to a twisting force on the tiller arm as the steering mechanism changes from a horizontal position to a vertical one. This change in motion and steering force results in an inconsistent operation, level of steering assist, and ‘feel’ to the operator depending on the relative angle and position of the tiller arm and steering handle.
The present invention addresses these issues.